A DETAILED READOUT OF THE FLOW PATTERNS IN CAROTID ARTERIES WILL HELP DOCTORS IDENTIFY WHO'S AT HIGH RISK FOR STROKE.

If the brain is a light bulb and blood to the brain is electricity, then
a stroke is lights out, what happens when you cut off the flow. Strokes
kill about 150,000 Americans each year. They're the third-leading cause
of death in this country, after heart disease and cancer, and the
leading cause of adult disability, affecting two to three million
American stroke survivors.

The Carotid Artery from Gray's Anatomy

Most strokes - about 80 percent of 700,000 a year in the U.S. - happen
because an artery that carries blood uphill from the heart to the head
gets clogged. Most of the time, as with heart attacks, the problem is
atherosclerosis, hardening of the arteries, calcified buildup of fatty
deposits on the vessel wall. The primary troublemaker is the carotid
artery, one on each side of the neck, the main thoroughfare for blood to
the brain.

Awareness of a relation between strokes and the carotid artery is at
least as old as the name - from the Greek verb karoun, to plunge into
deep sleep or stupor. Only within the last 25 years, though, have
researchers been able to put their finger on why the carotid is
especially susceptible to atherosclerosis.

"Blood has the same level of cholesterol in our toes as in our coronary
arteries," says Frank
Loth, a biomechanics professor at the University of Illinois,
Chicago, "so you might expect that atherosclerosis would be a diffuse
disease, that we'd get it anywhere. But we don't. There are particular
sites - coronary arteries, abdominal aorta, carotid arteries and
others."

Frank Loth (middle) and graduate student Seung Lee, University of
Illinois, Chicago, and Hisham Bassiouny, University of Chicago
Hospital.

Loth's specialty is hemodynamics, fluid dynamics of the blood - a
relatively recent and growing field of work that has produced some
answers about atherosclerosis. For the past 15 years, Loth has teamed
with University of Chicago vascular surgeon Hisham Bassiouny to study
vascular hemodynamics. Among other things, they've worked on
understanding flow in the carotid artery, both healthy and with arterial
narrowing - called stenosis - due to the plaque buildup of
atherosclerosis.

"We're trying to define the hemodynamics for different degrees of
carotid stenosis," says Bassiouny, who specializes in carotid artery
disease and in endarterectomy, a life-saving surgical procedure to
remove plaque from the carotid artery. "Our hypothesis is that there are
specific flow patterns, turbulent and non-turbulent, that may predispose
to plaque progression or plaque breakdown."

Paul Fischer (left) and Henry Tufo, Argonne National Laboratory.

Four years ago, Loth and Bassiouny joined forces with two computer
scientists, Paul Fischer of Argonne National Laboratory and his
associate Henry Tufo, experts in the numerical methods of flow modeling.
The objective: Develop the ability, with computational modeling, to
provide a detailed readout of the flow patterns and forces in the
carotid arteries of patients, information that doctors can then use to
help identify who's at high risk for stroke.

With a mix of disciplines to fit the job - vascular surgery, fluid
mechanics, advanced numerical methods - the Chicago-based team has made
rapid strides. This year, with availability of LeMieux, Pittsburgh Supercomputing Center's
terascale system, they've done what hasn't been done before. Starting
with a CT scan from a patient's severely clogged carotid artery, they've
simulated the transition from smooth to turbulent flow that occurs in
that artery over the course of one heartbeat. Just as importantly,
they've demonstrated that it's feasible to produce this kind of
information quickly, within 24 hours, so it can be used in treatment
planning.

Shear Stress

Over the last 20 years, hemodynamics has established a relation between
flow patterns and the likelihood of atherosclerosis. The vessel sites
most susceptible to disease are like the outside bank of a stream where
there's a sharp turn. "You might have a region where water is slow,"
says Loth, "and you'll see leaves and branches in a recirculation area
with a little sandy beach. The same thing happens in arteries."

When there's low flow velocity and recirculation, the vessel wall feels
"low shear stress." Like the force you exert on a desktop as you slide
your hand across it, shear stress is force in the direction of flow. Low
shear stress, research has shown, is one of the key factors in
predicting whether someone with healthy arteries will develop
atherosclerosis.

In the carotid artery, low shear stress tends to happen near a
particular site - the carotid bifurcation - where the artery splits in
two. In one branch, just past the fork, a healthy artery is spacious and
then narrows as it turns inward toward the brain. In the spacious
region, flow along the outer wall is often slow with recirculation,
prime territory for trouble.

Over time, as plaque builds up, the flow patterns at this site change.
In a healthy, spacious artery, the flow is smooth. In a stenosed, narrow
artery, flow into the bend is faster and, with enough narrowing, becomes
turbulent. The increased force of this flow can disrupt plaque, a
potentially fatal problem. "The mechanism of a stroke in half the
cases," says Bassiouny, "is plaque in the carotid artery that breaks
apart. As fragments travel upstream, they can block a vital artery."

The choice of treatment for carotid atherosclerosis - blood-thinning
medication, surgery, or no treatment - depends not only on the degree of
narrowing, but also on whether the plaque is likely to fragment. Knowing
the flow patterns and forces, says Bassiouny, would lead to better
decisions. "Not every patient who has plaque has a stroke. For someone
with 60 percent stenosis, we could decide the case is non-conducive to
progression and instability. Another patient with 60 percent stenosis
but with different plaque configuration and flow dynamics might need an
endarterectomy."

Turbulence and Spectral Elements

Click Images to Enlarge

A Carotid Artery Snapshot: From Scans to Computational Mesh
From CT scans at cross-sectional slices along a patient's carotid
artery, the researchers construct a computational mesh, which
becomes the framework for highly detailed simulations. The narrow
region of the internal branch of this artery (orange), just past the carotid bifurcation, is the
stenosed region, where 70 percent of the healthy flow
cross-section is blocked. In a healthy artery, this region is
larger than the downstream region after the artery turns.

The flow in healthy carotid arteries has been simulated before, but
there's good reason why it hasn't been done until now in a stenosed
artery. Turbulent flow is much more complex and greatly complicates the
numerical problem, multiplying the demand on computing at least 100
times, well beyond the ability of most hemodynamics software and
workstation computers.

Fischer is a pioneer in an advanced numerical approach called "the
spectral element method," having worked on his dissertation at MIT with
its originator, Tony Patera. The advantage is high accuracy with
efficient use of computing resources. In 1999, a major computing award,
the Gordon Bell Prize, recognized Fischer and Tufo for the quality
algorithm and fast performance of Nek5000, their spectral element
program.

Loth and Fischer spent the first two years of their collaboration
adapting Nek5000 to simulate vascular hemodynamics. Tufo is a specialist
in "scaling," software techniques to maximize the teamwork among
hundreds or thousands of processors in massively parallel systems, and
he took charge of getting Nek5000 up and running efficiently on LeMieux.
With his fine tuning and using 2,048 processors in test runs, Nek5000
steps through its paces at 1.25 trillion calculations per second.

Bassiouny provided CT scans from a 55-year-old man with a 70 percent
stenosed carotid artery. With the scan data as input, Seung Lee, a
University of Illinois, Chicago graduate student who works with Loth,
used a series of programs to construct a mesh-like computational grid.
Doppler ultrasound measurements from the patient provided the initial
flow velocity.

Using 256 processors for this first real-case simulation, the
researchers were able to simulate a full cardiac cycle - one heartbeat -
in 11 hours of wall-clock time. An animation
depicts the results at a cross-sectional slice through the artery, as if
looking down at a river in which the flow alternately rushes forward and
then slows as the heart relaxes. Just around the bend from where the
carotid turns inward toward the brain, as the flow feels the force of
the heart's contraction, a slow, lazy river transforms to a torrent with
violent swirls of turbulence.

Click Images to Enlarge

Transition to Turbulence in a Stenosed Carotid Artery
These snapshots from the simulation show increasing turbulence as
the flow approaches the systolic peak, when it feels the force of
the heart's contraction. The complex twisted structures (seen in
closeup of the last snapshot) are vortex surfaces, a way to
visualize the structure of turbulent flow, with color (red to
blue) indicating pressure decrease. The pressure drops markedly,
corresponding to increased flow rate, in the stenosed region
(before the turn), and the high flow rate and low pressure
continue around the bend.

It's the first time this transition has been captured by simulation of
an actual patient's carotid artery. Fischer is pleased not only with the
flow results but also with the computing turnaround. "I didn't think
we'd be in this 24-hour range on our first shot. We're ahead of the
curve for two reasons: the numerical methods we use and having access to
a machine like Pittsburgh's."

THE TOOLS ARE IN PLACE TO ZERO-IN ON THE CORRELATIONS BETWEEN TURBULENT FLOW AND STROKE.

Part of the objective for this first round of simulation was to measure
how rapidly flow velocity fluctuates with time, from which the
researchers can judge how thinly to slice the calculations in the next
round of 10 cardiac cycles. At the turbulence peak, the midstream flow
velocity fluctuates at about 350 cycles per second, which means they'll
need to take a computational snapshot every thousandth of a second to
capture details of the flow.

It's the first good look at the transition to turbulence in a carotid
artery, and along with new information, it demonstrates that tools are
in place to zero in on the correlations between turbulent flow and
stroke. "Within five years," says Fischer, "it should be possible to
routinely simulate weakly turbulent hemodynamic flows." For medical
researchers, this means it's feasible to gather flow data on a range of
patients with diseased carotid arteries and carry out long-term studies.
What degree of turbulence and high shear stress under what conditions
means serious risk of stroke? Getting the answers is now within sight.

Visualizations:
Simulation frames courtesy of Wojtek Kalata, University of Illinois,
Chicago and Mick Coady, University of Colorado, Boulder using software
developed by the Futures Laboratory in the Mathematics and Computer
Science Division at Argonne National Laboratory.

The Pittsburgh Supercomputing Center is a
joint effort of Carnegie Mellon University and the
University of Pittsburgh together with the Westinghouse
Electric Company. It was established in 1986 and is
supported by several federal agencies, the Commonwealth of
Pennsylvania and private industry.